There are several applications for lasing radiation pulses having a duration in the picosecond regime including ultra-short time resolving spectroscopy. For most applications, it is desirable to produce stable and reproducible ultra-short lasing radiation pulses. By stable pulses it is meant pulses that are essentially a single well defined pulse. However, by referring to stable pulses it is not intended to preclude the possibility of single well defined pulses which may be followed by relatively small damped relaxation oscillations or ringing. A pulse would not be classified as stable if it is followed by damped relaxation oscillations that have an amplitude in excess of a significant percentage of the maximum amplitude of the initial or single well defined pulses. Reproducibility of the pulses is the requirement that all the pulses are stable pulses notwithstanding variations in the power or duration of the incoming pumping pulses.
In the past, laser systems using mode-locking have been used to produce pulses having a duration of approximately 10 picoseconds or less. However, these systems are very expensive, have low output power, require considerable maintenance and have a limitation in their output wavelength.
In the Journal of Applied Physics, Volume 37, No. 5 (1966) 2004, D. Roess, developed a "Resonator Transient" theory. This theory proposed the method of pulse shortening by pumping a laser oscillator or cavity having transient characteristics such that the resulting pulse is shorter than the pump pulse.
Hereinafter the duration of a pulse will be taken to mean the commonly accepted definition of full width half maximum. That is to say, the duration of that portion of the pulse which is in excess of one half of the maximum amplitude of the pulse.
H. Salzmann and H. Strohwald, Physics Letter 57A (1976) 41 report emission of 10 picosecond pulses from a longitudinally pumped dye laser cell with a 0.04 picosecond photon cavity lifetime. Because of the extremely short photon cavity lifetime, single pulses could only be obtained when pumping was just above the threshold. Threshold is the minimum pumping energy required to produce a lasing action in the dye laser cavity. A further example of the difficulty involved in the pumping of a single stage dye laser cavity is reported in E. Aussenegg and A. Leitner, Optics Communication, Vol. 32 No. 1, p. 121. For resonator lengths of less than 0.5 mm, which resulted in a photon cavity lifetime of approximately 8 picoseconds, an irregular sequence of relaxation pulses was observed. With a resonator length of 0.5 mm the duration of the output pulses from the dye laser cavity was approximately 50 picoseconds.
In the Salzmann and Strohwald experiments the dye laser cell was pumped by a high pressure nitrogen laser producing pulses having a duration of approximately 100 picoseconds. In the Aussenegg and Leitner experiments the dye laser cavity was pumped with pulses having a duration of approximately 300 picoseconds. Aussenegg and Leitner note that if the pump pulses were shortened by raising the nitrogen laser pressure, a corresponding reduction of the dye laser resonator length should yield pulses shorter than 10 picoseconds. However, there are attendant problems with generating short duration pumping pulses by increasing the pressure in the nitrogen laser. Practical nitrogen lasers have an output duration of approximately 300 picoseconds or more. Eximer lasers having the same physical configuration as the aforementioned nitrogen lasers are also excellent pumping sources but they have output durations of approximately 1 nanosecond.
In practice, the duration of the pumping pulses can vary over a broad range. This variation has made it impractical to produce stable and reproducible ultra-short lasing radiation pulses emerging from just one dye laser cavity. As Aussenegg and Leitner note, Salzmann and Strohwald could only obtain single pulses when the cavity was pumped close above threshold with their extremely short photon cavity lifetime. Variations in the pumping pulses would produce washable pulses.
In the IEEE Journal of Quantum Electronics, Volume QE-11, No. 8, August 1975, Shinlon Lin reports a study of relaxation oscillations in organic dye lasers. He demonstrates that when the ratio of the pumping power to the threshold power of the dye laser is decreased, the output from the dye laser goes from uncontrolled spiking to a sequence of well defined damped relaxation oscillations to a single spike output. Consistent with the Aussenegg and Leitner experiments, he shows that with the choice of particular parameters for the dye laser cavity a single spike of a duration considerably shorter than than of the pumping pulse can be produced by carefully controlling the pumping power.
Therefore, in the past it has been impossible to reliably produce stable ultra-short lasing radiation pulses having a duration of less than 50 picoseconds other than by the mode-locking method. This is due to the fact that as the ratio of the duration of the pumping pulse to the photon cavity lifetime of the dye laser cavity becomes too large, stable pulses can only be obtained when the pumping power is very close to threshold. This critical limitation on the pumping power, coupled with the fact that each pumping pulse might not have exactly the same power and duration, has made it impractical to produce stable and reproducible ultra-short lasing radiation pulses of duration less than 50 picoseconds in the past.
If only a single dye laser cavity is used it is impossible to reliably produce ultra-short lasing radiation pulses having a duration of less than 50 picoseconds when the cavity is pumped by pulses having a duration in excess of 300 picoseconds. If the photon cavity lifetime is chosen so that sufficiently short pulses are produced, the resultant pulses will not be stable and reproducible. In the past, the use of a single dye laser cavity has resulted in a trade-off between reproducibility and shortness of duration, both factors being impossible to achieve simultaneously.
One example of a cascade pumping scheme is reported by K. Kato in "IEEE Journal of Quantum Electronics", July 1976, 442. A first dye laser was pumped in an off axis longitudinal pumping scheme. The lasing radiation emerging from this dye laser was directed into a second dye laser, also in the off axis longitudinal pumping scheme. The first dye laser cavity was 30 cm long and the second dye laser cavity was 40 cm long. This arrangement was not for the purpose of producing ultra-short lasing radiation output pulses but rather was for the purpose of matching the pump wavelengths to the principal absorption hands of the dye compounds used in the dye lasers. The utility of the cascade pumping scheme was clearly demonstrated by the fact that about a 9 percent overall energy conversion efficiency was obtained whereas direct pumping of the dye in the second dye laser cavity resulted in an energy conversion effeciency of less than 1 percent. Although cascade pumping schemes have been used in dye laser arrangements in the past, they have been used for the purpose of overall energy conversion efficiency and flexibility in the output lasing radiation wavelength rather than for the purpose of producing stable and reproducible ultra-short lasing radiation pulses. The present invention uses a cascaded pumping scheme for dye laser cavities where the photon cavity lifetime of each dye laser cavity is chosen so that stable and reproducible ultra-short lasing radiation pulses emerge from the final stage.